Errata
Document Title: Schottky Barrier Video Detectors (AN 923)
Part Number: 5954-2079
Revision Date: November 1999
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Schottky Barrier Diode
Video Detectors
Application Note 923
I. Introduction
This Application Note describes the characteristics of Hewlett-Packard
Schottky Barrier Diodes intended for use in video detector or video
receiver circuits, and discusses some design features of such circuits.
Although a video receiver is typically 35 - 40 dB less sensitive than a
heterodyne receiver, nevertheless its simplicity and low cost can
outweigh this disadvantage. For example, a video receiver requires no
local oscillator power; it generally uses only a single diode; it is capable
of being built with a large RF bandwidth; and it is much less critical to
design and maintain than a heterodyne receiver. These advantages
make it useful as beacon receivers, missile guidance receivers, fuseactivating receivers, countermeasures receivers, and power leveling
and signal monitoring detectors.
A typical video receiver circuit is shown in Figure 1. The operation of
this circuit is quite simple. At RF, the bypass capacitor (Cb) appears as
a short circuit, and the input RF choke (RFC1) appears as an open
circuit. The input RF filter is optimized to match the signal source
impedance to the diode’s RF impedance over a specified bandwidth
and, ideally, all of the available signal power is delivered to the diode.
Due to the nonlinearity of the diode, the video signal is extracted from
the modulated RF signal and appears across the video load resistance
(RL). At video frequencies, RFC1 acts as a short circuit, while Cb and
BIAS CURRENT
SOURCE
Cc
Rs
RF FILTER
AND
IMPEDANCE
TRANSFORMER
e1
SIGNAL
SOURCE
DETECTOR
DIODE
1
RFC1
RFC2
3
Cb
RL
CA
VIDEO
AMPLIFIER
4
2
RF PORT
Figure 1. Typical Video Receiver
VIDEO PORT
2
the DC bias filter, consisting of RFC2 and Cc, both appear as high
impedances. RFC1 also serves as a return path for the DC bias current.
The principal requirements for a video receiver are distortionless
recovery of the modulation signal, which is usually a pulse, and
maximum RF sensitivity. However, these requirements are competitive
and result in the design being, at best, a good compromise between
video bandwidth, which determines the fidelity of the detected pulse,
and RF sensitivity.
II. Diode Performance Characteristics
The performance characteristics that are used to describe video
detector diodes are Tangential Sensitivity, Video Resistance, and
Voltage Sensitivity.
1. Tangential Sensitivity (TSS) – This is the lowest signal power
level for which the detector will have a specified signal-to-noise ratio at
the output of the video amplifier. At Hewlett-Packard, the output signal-to-noise ratio is specified to be 8 dB [1]. The units for TSS are dBm
or milliwatts. As we shall see later, TSS does not depend entirely on
intrinsic diode parameters and many factors affect the measured TSS
value for a given diode. For the time being, the most important factors
are:
a) RF Frequency
b) Video Bandwidth
c) Diode DC Bias Current
d) Test Mount or Circuit
e) Video Amplifier Noise Figure
For an exact TSS specification, the effective video bandwidth should
be stated as the lower and the upper 3 dB frequencies of the entire
video circuit, including the diode’s video resistance. A statement of
only the bandwidth of the video amplifier can be misleading because it
does not always determine the overall or effective bandwidth of the
system. The limitation on overall bandwidth can come both from the
circuit between the diode output and the amplifier input or the
instrumentation circuit after the amplifier, i.e., oscilloscope or meter.
Because the upper 3 dB frequency is usually several orders of
magnitude greater than the lower 3 dB frequency, it is common to state
only the upper 3 dB frequency. For example, a usual statement of video
bandwidth may be 2 MHz. The implication is that the response of the
video circuit of the detector to the modulating signal extends from DC
to 2 MHz. In actual practice, the low frequency response very seldom
extends down to DC, because this will include the flicker noise
contribution of the diode and of the video amplifier, both of which
deteriorate TSS. If the diode is expected to be used in a system that
will require low frequency response, then the low frequency 3 dB point
of the test system must be stated. Failure to do so can lead to gross
differences in TSS between the test system and the actual system if the
diode’s flicker noise corner frequency fN is high, i.e., >50 kHz.
To obtain maximum sensitivity at any given frequency, most [2]
3
detector diodes must be forward DC biased. Bias however introduces
shot and flicker (I/f) noise in the diode, and reduces the diode video
resistance. These effects exert a competitive influence on TSS –
therefore, the bias value must be stated.
Since diodes of different designs can differ widely in their RF
impedance characteristics, particularly if package parameters are
different or are not sufficiently well controlled, repeatability of
performance can only be obtained by a test in a specific mount and at a
specific frequency. At HP, detector diodes of different types are tested
in mounts that have been tuned and “locked” to that type.
2. Video Resistance (Rv) – This parameter is simply the small signal
low frequency dynamic resistance of the diode and is dependent on the
DC bias current. The value of the bias current used is the same that is
used in the TSS test.
Rv consists of the sum of the diode’s series resistance (Rs) and the
junction resistance (Rj)
RV = Rs + Rj
(1)
Rj is obtained by differentiating the diode voltage-current characteristic
and is given by:
Rj (i) =
nkT
q(Id + Is)
(2)
where Id is the bias current and Is is the saturation current. Is is ≅ 10-9
amperes for HP detector diodes and is negligible when the diode is
operated in the optimum bias region.
The nonideality factor n is different for each diode type. Typical values
for HP detector diodes are:
Diode Type
HP 5082-2750
HP 5082-2824
n
1.08
1.02
For an n = 1.08 and at room temperature, Rj can be simplified to:
Rj (i) ≅
28
Id
(3)
where Id is in milliamperes and Rj is in ohms.
3. Voltage Sensitivity (γ) – This parameter specifies the slope of the
output video voltage versus the input signal power, i.e., Vo = γPin of the
diode. It is bias, load resistance, signal level, and RF frequency
dependent, and all of these conditions must be specified. It is particularly sensitive to signal level which must be kept well within the
square law dynamic range of the diode.
4
III. Optimum Video Detector Sensitivity
Assuming a diode ideality factor n = 1.08, and at room temperature, the
TSS of a video receiver can be stated as [3]:
(4)
TSS(dBm) = -107 + 5 log BV + 10 log Id + 5 log RA + 28
Id
+ 10 log 1 +
fN
BV
1n
BV
fL
RSC2j(i) f2
Id
TEST CONDITIONS:
=
=
=
=
=
=
VIDEO LOAD IMPEDANCE (RL)
VIDEO BANDWIDTH (BV)
EQUIVALENT NOISE RESISTANCE
OF VIDEO AMPLIFIER (RA)
Video bandwidth in Hz
Diode dc bias in µA
Diode flicker noise corner frequency, Hz
Video circuit low frequency 3 dB point, Hz
Diode series resistance, Ω
Diode junction capacitance, pF, at the bias current Id
For the HP 5082-2750 and 5082-2755 diodes, this is approximately
f
RA
=
=
1 – 0.1 log (1300 Id)
(5)
Operating frequency, GHz
Amplifier equivalent series noise resistance, kΩ
This expression reveals that the only significant diode parameters that
affect the detector sensitivity are the three parasitic parameters fN, Rs,
and Cj. It can also be shown that there is a value of bias current, for a
given diode and operating frequency, which results in maximum TSS.
Assuming that the video amplifier contributes negligible noise
compared to the diode, RA can be neglected in Eq. (5). Differentiating
this simplified expression yields an approximate value of the optimum
bias as:
Id(OPT) (µA) ≅ RS(Ω) [Cj(i) (pF)]2 [f(GHz) ]2
(6)
which is valid for 0 < Id < 50 µA.
The effect of bias on the TSS of several HP detector diodes is shown in
Figure 2, and the same effect at different signal frequencies for the HP
5082-2755 is shown in Figure 3.
It is worthwhile to assess the magnitude of the degradation on
sensitivity that is introduced by the various factors in Eq. (5).
Assuming the following parameters for the system and the diode:
500 Ω
-56
-54
RF =
10 GHz
-52
HP 5082-2755
HP 5082-2750
HP 5082-2824
RF = 2 GHz
-50
-48
-46
5
10
100
DC BIAS CURRENT (µA)
500
Figure 2. Effect of DC Bias on TSS
TSS – TANGENTIAL SENSITIVITY (dBm)
Cj(i) =
Cj(o)
38 kΩ
2 MHz
-58
TSS – TANGENTIAL SENSITIVITY (dBm)
where
Bv
Id
fN
fL
Rs
Cj (i)
1+
TEST CONDITIONS:
BV = 2 MHz
RA = 500 Ω
RL = 38 kΩ
62
3 GHz
61
4 GHz
60
6 GHz
59
8 GHz
58
57
10 GHz
56
55
12 GHz
54
53
52
15.5 GHz
51
1
2
3 4 56
10
20 30 40 60
DC BIAS CURRENT (µA)
Figure 3. Effect of Bias on TSS at
Different RF Frequencies (Typical
5082-2755)
100
5
We get the following:
10 log 1 +
RSCj(i)2f2
= 3.0 dB
Id
5 log RA + 28 1 + fN 1n BV
Id
BV
fL
≅ 1.4 dB
,,,
,,,
50
NOISE TEMPERATURE RATIO (dB)
Diode and System Parameters
Diode HP 5082-2750:
fN = 3 kHz, Id = 20 µA, f = 10 GHz,
RA = 0.5 kΩ, Bv = 2 MHz, Rs = 25 Ω
TYPICAL POINT CONTACT DIODES
(BIASED FOR OPTIMUM TSS)
40
TYPICAL
HP 5082-2750
2755, 2824
30
20
10
DC BIAS =
20 µA
0
-5
102
= 13.0 dB
5 log BV
= 31.5 dB
Total Degradation
= 48.9 dB
Expected TSS = -107 + 48.8 = -58.1 dBm
This is typical of the HP 5082-2750 diode which is specified at -55 dBm
to allow for reasonable variations in diode parameters. The
significance of the above is the relative magnitude of the various
degradation factors in a particular system. The degradation due to
diode flicker and shot noise is particularly low due to the low flicker
noise corner frequency for this diode when it is operated at the
optimum bias of 20 µA. The typical flicker noise characteristics of the
HP 5082-2750 and 5082-2824 diodes are shown in Figures 4 and 5.
Figure 4 shows the flicker noise characteristic as a function of video
frequency at 20 µA bias, and Figure 5 shows the change in the noise
corner frequency fN as a function of bias. Even at high bias levels, fN is
sufficiently low, i.e., 30 kHz, by comparison to normal video
bandwidths, typically > 300 kHz, that the flicker noise contribution can
be considered negligible. This can be seen in Figure 6 in which the
factor
103
104
105
106
FREQUENCY (Hz)
107
Figure 4. Flicker Noise
Characteristics of HP Detector Diodes
100
fN – NOISE CORNER FREQUENCY (KHz)
10 log Id
TYPICAL
fN = 1 MHz
fN =
3 kHz
10
1
100
10
BIAS CURRENT (µA)
1
1000
Figure 5. Noise Corner Frequency vs.
Bias (Typical HP 5082-2750, 50822755)
5 log RA + 28 1 + fN 1n BV
Id
BV
fL
of Eq. (5) is plotted as a function of Bv with fL fixed at 100 Hz. For
most applications of the HP Detector Diodes, this factor can be
simplified to:
5 log RA + 28
Id
(7)
∆TSS – TANGENTIAL SENSITIVITY (dB)
10.0
FN = 1.0 MHz
FL = 0.1 KHz
8.0
6.0
4.0
2.0
FN = 0.1 MHz
FN = 0.01 MHz
FN = 0.003 MHz
0
0.1
0.2
2.0
4.0 8.0 10.0
0.4
1.0
VIDEO BANDWIDTH (MHz)
Figure 6. Amplifier and Diode Noise
Contribution as a Function of Video
Bandwidth
6
IV. Bandwidth Requirements
To maintain high sensitivity, the video bandwidth of the video detector
should be no greater than necessary to recover the modulation
information. The bandwidth required for pulse recovery depends on
the nature of the information to be gained from the pulse. For example,
in pulse search radar, peak pulse detectability is more significant than
pulse shape. In ferret systems, instrumentation, or data transmission
systems, the resolution of the fine details of the pulse shape, i.e.,
risetime and pulsewidth, may be of much greater importance in spite of
greater bandwidth requirements.
For maximum pulse detectability, the video bandwidth should be made
just sufficient enough to maximize the output signal-to-noise ratio of
the detecting system. This condition occurs when the video circuit
transfer function is matched to the spectrum of the signal
waveform [4,5]; i.e., for a rectangular pulse and a rectangular low pass
filter response, the required filter bandwidth is
BV ≅ 1/tw
where
(8)
tw = Pulsewidth
For a simple RC filter, which is the usual case at the output of the
detector diode, the maximum S/N occurs when the filter’s upper 3 dB
video frequency is approximately:
fu(3 dB) ≅
0.25
tw
(9)
In other applications, it may be desirable to resolve the shape of the
pulse, i.e., risetime, pulsewidth and flat amplitude. To resolve the
risetime of the pulse, the upper 3 dB frequency of the video circuit will
have to be
fu(3 dB) ≅ 0.35/tr
where
(10)
tr = Pulse risetime
To resolve the shape of the pulse to any degree of accuracy,
consideration must also be given to how much amplitude droop is
permissible. Amplitude droop depends on the low frequency 3 dB point
and is given by:
fL(3 dB) =
Droop %
600 tw
(11)
7
V. Design Considerations for Video Detector
The basic video detector circuit and its equivalent representations at
both the RF and video ports are shown in Figure 7.
DIODE
DC BIAS
CP
LP
RS
RJ
RF
PORT
Cb
CJ
RL
VIDEO
PORT
CA
(a) COMPLETE VIDEO DETECTOR CIRCUIT
LP
RS
RV
RF
SIGNAL
CP
CJ
Cb
RJ
RL
CA
eV
RV + RS + RJ
Cb
(b) EQUIVALENT CIRCUIT AT
THE RF PORT
(c) EQUIVALENT CIRCUIT AT THE
VIDEO PORT
Figure 7. Video Detector Equivalent Circuits
(A) Video Circuit Design Considerations
In the video circuit, RL represents the load or amplifier input
resistance, and CA represents the amplifier input capacitance as well as
all the stray and particularly cable capacitances that may be present in
the video circuit. These R and C elements will impose a limit on the
upper 3 dB cut-off frequency of this circuit, which is given by:
fu(3 dB) =
where
and
1
2πRTCT
RT =
(12)
RVRL
(13)
RV + RL
CT = Cb + CA
(14)
In most video detector designs, this circuit has a greater influence on
the effective video bandwidth than the bandwidth of the amplifier. The
RTCT time constant can be reduced by reducing all the element values
within certain limits. A severe reduction in the value of the RF bypass
capacitance Cb will lead to poor RF/video isolation and a decrease in
the signal level delivered to the diode. The reactance of this capacitor
at the operating frequency should be kept to less than 10% of the RF
impedance of the diode. At low RF frequencies and wide video
8
bandwidths, this capacitor can be replaced by a low pass filter
structure as shown in Figure 8 with a cut-off frequency fc ≤ 1 / 2 frf .
Because this filter is required to pass fast pulses, the design should be
for flat time delay rather than for equal-ripple.
A reduction in stray and amplifier capacitances is always advisable and
in wideband designs is often necessary. One suitable technique which
is particularly effective when cable interconnections must be made
between the detector circuit and the video amplifier is shown in Figure
9. In this circuit, the original cable capacitance Co is reduced by
feedback to: Ceff = Co (1 - Av) where Av is the total gain from the input
of the amplifier to the first shield and must be < 1.
TRIAXIAL CABLE
TO
DETECTOR
TO FEEDBACK
LOOP
Figure 9. Circuit for Reducing Cable Capacitance
Alternately, either RL or Rv can be reduced. The amount that RL can be
reduced is often limited if voltage amplification is desired since the
output voltage of the detector is maximized by making RL large. Rv of
the diode can be lowered by increasing the bias current. Although this
results in reduced sensitivity, as was shown in Figure 2, it may
nevertheless be needed to achieve the required video bandwidth. A
reduction of video resistance has other beneficial effects. It can, for
instance, be adjusted to be the optimum source resistance value for a
minimum noise figure of the video amplifier. When this is desired, Rv
should be adjusted so that
RV ≅
RNSRNP
(15)
where RNS = Equivalent series noise resistance of amplifier
RNP = Equivalent parallel noise resistance of amplifier
This is usually in the range of 400 - 2000 ohms for most low noise
transistor input stages.
RV
Figure 8. Low Pass Video Coupling
Structure
9
(B) RF Circuit Design Considerations
The RF circuit consists essentially of a filter structure that is designed
to match the signal source resistance, usually 50 ohms, to the diode
junction resistance, Rj. The equivalent circuit of the diode at RF is
shown in Figure 7b. The presence of RS and Cj introduces a loss of
signal which is dependent on frequency and the magnitudes of RS, Cj,
and Rj. This dependence is given by
RS
Ldb = 10 log 1 +
where
Rj
+ ω2 Cj2 RSRj
(16)
ω = 2πf
Since Rj is a function of bias current, i.e., Rj ≅ 28/Id , the above can be
restated as
28ω2 Cj2 RS
(17)
Id
from which it is obvious that the RF loss can be minimized at any given
frequency by biasing the diode. This effect was included as one of the
bias dependent parameters in the TSS expression (Eq. 5), which was
then used to determine optimum bias with respect to maximum TSS at
a given frequency. Bias also affects the RF impedance of the diode, and
the impedance that is obtained at a bias corresponding to maximum
TSS is not necessarily optimum for achieving broadband RF matching
to the diode.
.8
1.0
.6
1
Rj
.2
1.5 3 4 5
10
-10
0
8 GHz
.2
.4 .6 .8 1.0
TYPICAL
HP 50822750
12 GHz
2 GHz
2 GHz
4 GHz
6 GHz
10 GHz
8 GHz
-.6
-.8
0
jωCj +
12 GHz
10
(18)
1
4 5
RS + jωLP +
3
1
2
10 GHz
-.4
Y12 = jωCP +
.4
-.2
The typical measured RF impedances for HP 5082-2750 and 5082-2755
diodes, which are the same chip in two different packages, are shown
as a function of frequency in Figure 10. Figure 10 shows the diode
when it is biased at 20 µA (Rj ≅ 1400), which is its optimum bias for
maximum sensitivity at 10 GHz. As can be seen, the SWR for either
package style is high over most of the frequency range and normally
either diode will have to be matched to the source with additional
reactive elements. Practical reactances, however well made, will
introduce additional losses in the matching structure. Furthermore, the
farther these reactances are placed from the diode chip, the more loss
will be introduced due to high standing waves and current maxima
points due to high standing waves and current maxima points between
them. Even well made double-stub tuners can introduce losses as high
as 1.5 - 2 dB. The impedance plots of Figure 10 also clearly illustrate
the effect that package and chip parasitics can have on the RF
impedance of the diode. Assuming that the RF bypass capacitor Cb is
properly chosen and can be neglected, which was the case for the
impedance plots, the RF admittance of the remaining circuit can be
expressed as
1.5
TYPICAL
HP 5082-2755
-5
28
+
6 GHz -1.5
-1.0
-4
RSId
-3
Ldb = 10 log 1 +
4 GHz
-2
Figure 10. Typical RF Impedance of
HP 5082-2750 and 5082-2755 Diodes
at Bias Current of 20 µA
10
This admittance has two potential resonant frequencies. Neglecting RS
and Rj , these resonant frequencies can be expressed approximately as:
1
(19)
LPCj
1
1
+
LPCj
LPCP
(20)
As can be seen from the plots, a resonant frequency for the high
inductance glass package is evident at ≅ 7.9 GHz.
.8
3
TYPICAL
HP 5082-2755
10 GHz
4 5
.2
16 GHz
0
14 GHz
.4 .6 .8 1.0
10 GHz
6 GHz
1.5 3 4 5 10
2 GHz
2 GHz 16 GHz
6 GHz
-4
-3
-.4
-.6
-.8
-1.5
Figure 11. Typical RF Impedance of
HP 5082-2750 and 5082-2755 Diodes
at Bias Current of 330 µA
FREQUENCY - 2 GHz (HP 5082-2824)
FREQUENCY - 10 GHz (HP 5082-2750,-2755)
IBIAS = 20 µA
RL = 5 KΩ
LINEAR
2824
EOUT – VOLTAGE OUT
10V
SQUARE LAW
HP 27502755
1V
100 mV
10 mV
1 mV
100 µV
10 µV
As can be seen from Figure 12, the transfer characteristic of a Schottky
Barrier Diode does not simply saturate beyond this point but merely
changes from square-law to linear, which then continues to a higher
power level before saturation. This range is defined as the linear
dynamic range and can be appreciable if the breakdown voltage of the
diode is high. For example, the HP 5082-2824 detector diode is rated at
-2
-1.0
(21)
where γ is the diode voltage sensitivity at the specified frequency and
bias. Deviation from square-law characteristic occurs when γ departs
from a constant value. This will occur when the RF carrier and,
consequently, the rectified current become high enough to appreciably
affect the quiescent bias current, Id. At HP, the upper limit of squarelaw operation is defined to be the power level which is 0.5 dB higher
than the power level that would produce the same output voltage if γ
had remained constant.
0
.2
10
TYPICAL
HP 5082-2750
In addition to achieving high sensitivity, video detectors are usually
required to have a large dynamic range. Usually this statement refers to
the “square-law” range as shown in Figure 12 and is the range over
which the output signal voltage is proportional to the input signal
power, i.e.:
Vs = γPin
2
-.2
VI. Dynamic Range
1.5
.4
Due to the low values of the parasitic elements, the pill packaged
device 5082-2750 does not exhibit any resonances up to a frequency of
12 GHz. Such a diode can be brought into match over fairly broad
bandwidths by a series length of line and a quarterwave transformer [6].
To simplify broadband matching, the impedance of the diode can be
reduced by using more DC bias than would normally be required for
maximum TSS. Figure 11 illustrates the magnitude of the effect that
bias has on the RF impedance. These plots are made for the same
diodes as in Figure 9, but at a bias of 330 µA. The 5082-2750 pill
packaged diode impedance is now within a SWR of : 2/1 over most of
its useful frequency range. Although there will also be a reduction of
sensitivity of about 7 dB, as was shown in Figure 2, nevertheless, the
trade-off may be worthwhile in broadband systems.
1.0
.6
-10
ωP ≅
-5
ωS ≅
-60
-40
-20
0
20
PIN – POWER INPUT (dBm)
Figure 12. Detector Dynamic
Characteristics
40
11
a breakdown voltage of 15 V. By contrast, point contact diodes whose
breakdown voltage is generally low (2 - 3 V), do not exhibit a broad
linear range. For applications that require monitoring of high level RF
signals, i.e., radio controlled trigger circuits, high power monitors, or
RF proximity fuses, this range is very useful. Applications that are not
critical of the change in the transfer characteristics can utilize the very
large total (square-law and linear) dynamic range. Finally, in very
critical applications where high square-law range is necessary, the
video amplifier gain characteristics can be shaped. The range over
which such compensation can be achieved will depend very much on
the video bandwidth of the amplifier. For bandwidths of 2 MHz or less,
the square-law range can be so extended to about +10 dBm with an
accuracy of ±1 dB.
VII. Detector Diode Burnout
The destruction of the detector under high signal level conditions
merits some discussion. It has been the industry standard to rate
detectors according to energy burnout, usually stated as so many ergs.
Assuming that energy burnout can be defined sufficiently well, it still
remains a very dubious performance requirement to be imposed on the
detector. With the exception of possible static discharge which can
occur during handling of the diode, total energy stress will very seldom
be the kind of stress that a video detector is exposed to.
HP 5082-2824
RF = 2 GHz
PULSE WIDTH = µs
DUTY CYCLE = 10-3
DC BIAS = 20 µA
LOAD RESISTANCE = 38 KΩ
DIODE MOUNT TUNED TO THE DIODE
AT THE TSS POWER LEVEL
INCIDENT REFLECTOR OR ABSORBED PEAK
POWER (WATTS)
Video detectors are often designed for broad, RF bandwidths, thus they
can be exposed to high peak power radiation from local radar
transmitters. A better measure of the video detector burnout is a
measure of its capability to handle peak powers in the range of 1 or
more microsecond duration, and to expose the diode to this stress for a
period of time at a specified repetition rate. Because the diode is
generally biased to a junction resistance on the order of 1 - 2 kΩ, the
RF circuit, including the package parasitics, consists of a large
impedance ratio matching filter that matches the small-signal
impedance of the diode to the source impedance. As the signal level
increases, the impedance of the diode changes and produces a
mismatch with respect to the matching structure. This mismatch
results in a reflection of some of the incident RF power and only a
portion of the available RF power is actually dissipated in the diode.
The diode, in effect, protects itself. Regardless how much of the
incident power is reflected, eventually a power level can be reached
where the difference between the incident and the reflected power is
sufficient to destroy the diode. This is the peak power dissipation
capability of the diode. The amount of peak incident power the diode
can handle depends very much on the characteristics of the RF
structure between the source and the diode junction. Figure 13
illustrates the typical peak power handling capability of an HP 50822824 diode using a narrowband tuning structure tuned to the diode at
the small-signal (TSS) level. In very wideband systems, where no
tuning structures are used and where a tolerable input SWR at small
signal levels is obtained by biasing the diode to a lower impedance
level, the above situation does not apply. Here an increase in the
incident power level brings the diode closer to a matched condition
and, consequently, the diode’s dissipation ratings apply.
PINCIDENT
10
PREFLECTED
5
PABSORBED
10
5
INCIDENT PEAK POWER (WATTS)
Figure 13. Peak Power Handling
Characteristics
15
In some applications i.e., power monitoring and modulating, the CW
rather than peak power ratings apply. As previously stated, since the
impedance of the diode is a function of incident power, the amount of
power the diode can handle is greater than its CW dissipation
capability. This is shown in Figure 14 for a typical HP 5082-2824
detector diode.
HP 5082-2824
INCIDENT REFLECTED OR ABSORBED
CW POWER (WATTS)
7
RF = 2 GHz
LOAD RESISTANCE < 1Ω
DC BIAS = 0
DIODE MOUNTED IN SERIES WITH
THE INNER CONDUCTOR OF A 50 Ω
COAXIAL LINE AND IN FRONT OF
A SHORT CIRCUITED END
PINCIDENT
6
5
4
PABSORBED
3
2
PREFLECTED
1
1
2
3
4
5
6
INCIDENT CW POWER (WATTS)
7
Figure 14. CW Power Handling Characteristics
References
1. Hewlett-Packard Application Note 956-1, “The Criterion for the Tangential
Sensitivity Measurement.”
2. Hewlett-Packard Application Note 969, “An Optimum Zero Bias Schottky
Detector Diode.”
3. A.M. Cowley and H.O. Sorensen, “Quantitative Comparison of Solid-State
Microwave Detectors,” I.E.E.E. Transactions on MTT, Vol. MTT-14,
December 1966.
4. D. Povejsel, et al., “Airborne Radar,” D. Van Nostrand Co. Inc., 1961 Chapter
5, Section 10.
5. M. Schwartz, et al., “Communications Systems and Techniques,” McGrawHill Book Co., 1966, Chapter 2, Section 4.
6. Hewlett-Packard Application Note 963, “Impedance Matching Techniques
for Mixers and Detectors.”
For technical assistance or the location of
your nearest Hewlett-Packard sales office,
distributor or representative call:
Americas/Canada: 1-800-235-0312 or
(408) 654-8675
Far East/Australasia: Call your local HP
sales office.
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Europe: Call your local HP sales office.
Data Subject to Change
Copyright © 1986 Hewlett-Packard Co.
Obsoletes 5953-4405
Printed in U.S.A. 5954-2079 (5/86)